Stepper motors designed to rotate through a predetermined angle upon application of a sequence of voltage pulses are well known. Such stepping motors are useful in fields where accurate, discrete angular motions reliably responsive to a controlled sequence of pulses are desired. Applications involving frequent repositioning of a mechanical member to successively different positions are ideally adapted for being driven through a linkage by a stepper motor. Thus, stepper motors are commonly used in disc drives to position the head carrying actuator relative to the information storage disc. Pulsed instructions are converted directly to accurate rotational motions by the stepping motor.
A typical stepping motor construction involves a rotor comprising a permanent magnet and one or more magnetically permeable discs having a plurality of teeth on their periphery, and a stator preferably made from magnetically permeable laminations incorporating a number of poles, each pole having a plurality of teeth on the pole surface, and a coil of electrically conductive wire wound around its root. The geometric configuration and the magnetic fields are such that the attraction between rotor and stator teeth result in tangential forces on the rotor that change the angular alignment of the rotor relative to the stator.
Accurate positioning of the rotor relative to the stator is attained by a carefully planned sequencing of voltage pulses to the coils around each stator pole, known as an algorithm. The accuracy of the mechanical alignment between the rotor and the stator portions of a stepper motor is critical in view of the extremely small air gaps between the teeth of the rotor and the teeth of the stator poles. Failure to provide such extremely accurate alignment may result in unacceptably large stepping angle and hysteresis errors, thus adversely impacting production yields.
In conventional stepping motors, the stator comprises two end bells that pilot on the stack of laminations and capture the stack between them while providing support for two rotor bearings, one in each end bell. One of the conditions needed to provide an evenly distributed air gap between the teeth of the rotor and the teeth of the stator poles, the axis of the rotor must coincide as closely as possible with the axis of the lamination stack bore of the stator. Achieving such a condition is difficult due to compliance in the lamination stack and the precision with which the stack and end bells must be machined, which also adds cost to the manufacture of stepping motors.
The disc drive market demands greater speed, increased accuracy in positioning the transducer over the target track, and lower power consumption in the stepper motor. This can best be achieved by optimizing the rotor/stator tooth geometry and dimensioning with improved tolerance compliance, as well as by decreasing the gap between the rotor and the stator. The solution to this problem lies in improved control of rotor and stator fabrication, and the provision of better means for maintaining alignment between the axes of the rotor and stator in the stepper motor, while reducing cost.